Genetic factors in aggression, Flashcards
Genetic factors in aggression
If aggression has a significant genetic component, then seeing aggression in one twin should positively correlate with aggression in the other. Monozygotic twins should show a higher concordance rate than dizygotic twins as they share 100% of their genes.
MZ twins evaluation
Coccaro et al. (1997) compared monozygotic versus dizygotic twin pairs. In this study, twin pairs were examined for the concordance of criminal behaviour for both twins. The MZ twin pairs were found to have a 50% concordance whereas DZ twins were only 19%. This research supports the role of genetics in aggression as the twins that were genetically identical, i.e. 100% of the same genes, were more likely to display criminal behaviour than the twins that only shared 50% of the same genes.
This is backed up as using old Danish police records Christiansen (1977) demonstrated that levels of criminality showed a stronger correlation between identical twins – with the same genes – than between dizygotic twins. However criminality is not always the same as aggression.
Other theories such as social learning theory lead us to question whether aggression is purely genetic.
The Supermale theory
The Supermale theory- The basic difference between men and women lies in their genotype. Most people have 23 pairs of chromosomes and on these chromosomes are our genes. One pair of chromosomes decides whether we are male (XY) or female (XX). The Super-Male hypothesis (Sandberg, 1961) suggested the XYY Genotype leads to aggression.
The Supermale theory evaluation
The supermale explanation is socially sensitive. For examples, Court-Brown (1965) discovered 314 patients that had the XYY chromosome presentation and he put forward the view that these patients should remain hospitalised given their increased likelihood of aggressive behaviour.
Later research by Alice Theilgard (1984) did show that 16 men out of 30,000 sampled had the xyy gene and that these were slightly more aggressive and slightly less intelligent but this is such a rare mutation that it does not explain aggression in the general population.
The Warrior gene
The Warrior gene- MAOa is an enzyme which helps with the reuptake of neurotransmitters including serotonin. Humans with the MAOa L gene (L is for Low) have a lack of MAOa enzyme. Without this enzyme to recycle it the level of serotonin may become depleted.
Brunner (1993) undertook a famous study on the males in a large family from the Netherlands, whose members were displaying high levels of aggression. Five of the males were found to have the dysfunctional version of the MAOA gene. The females in the family were not affected by the genetic dysfunction. The MAOA gene is carried on the X chromosome, and as females have two X chromosome even if they possess the dysfunctional MAOA gene, their corresponding X chromosome is likely to have a functional MAOA gene, which has been found to be dominant. The female genotype presentation has the effect of preventing the issue of the flawed monoamine metabolism. Therefore this MAOA L variant of the MAOA gene may be a possible genetic marker of aggression. Study of a small number of men from one family (idiographic) so low population validity, also family so may have shared environmental factors such as diet.
The Warrior gene evaluation
Scientists such as Godar et al. (2014) have refined selective breeding by specifically removing (knock-out) certain genes in mice. This has been done with the MAOA gene. The MAOA knockout mice showed increased aggression and also lower levels of serotonin, demonstrating the relationship between genes and neurotransmitters. When the mice were given fluoxetine to raise their serotonin levels, their behaviour returned to normal.
This is supported by Stuart et al. (2014) who studied 97 men who had been involved in severe domestic abuse to their partners. They found the most violent men had the faulty MAOA gene. These men engaged in the highest level of physical and psychological aggression and inflicted the worst injuries on their partners.
Caspi et al (2002): Interaction of MAOA gene AND abusive childhood led to aggression. If boys with the MAOa – L gene suffered abuse in childhood , they were 3 times more likely to be aggressive when they reached adulthood. This suggests an interactionist cause of aggression such as identified in the Diathesis stress-model.
